Aluminum readily oxidizes in the presence of air (Eqn. (1)), or moisture (Eqns. (2 and 3)), rapidly forming a thin, strong protective oxide film on any exposed aluminum metal surface, including both liquid and solid surfaces.4Al+3O2→2Al2O3  (1)3H2O+2Al→Al2O3+3H2  (2)H2→2[H]melt  (3)
Because aluminum oxide is very stable thermodynamically, it is typically present in all aluminum alloys. Therefore, any furnace charge contains unavoidable amounts of alumina as a typical coating, constituting an exogenous inclusion source. During the mold filling of the casting process, aluminum oxides are formed when the free surface of the melt front contacts air and particularly when the liquid melt velocity produces turbulent flow. A distinction is often made between oxides pre-existing in the melting furnace, referred to as “old oxides,” and those created during mold filling, called “young oxides.” Campbell, J., Castings, Elsevier Butterworth-Heinemann, 2003; Q. G. Wang, C. J. Davidson, J. R. Griffiths, and P. N. Crepeau, “Oxide Films, Pores and The Fatigue Lives of Cast Aluminum Alloys”, Metall. Mater. Trans. vol. 37B (2006), pp. 887-895. For young oxides, the cause of entrainment has been described as “surface turbulence,” a reference to phenomena such as two or more flow fronts joining together (bifilm, flow marks, fold, and cold shut), a contraction of the surface area of a liquid (with folding of the oxide surface), or the passage of a bubble through the liquid.
Young oxides are more detrimental to material properties than old oxides. Because of the lack of wetting between oxide films folded dry side to dry side in young oxides, the entrained oxide unfurls during solidification and acts like a void or crack in the solidifying aluminum casting. These cracks can not only be initial sites for pore formation, but also be frozen into the solid and can significantly decrease the tensile and fatigue strengths of the casting. The bi-films can also cause hot tearing. Entrained oxides are believed to increase melt viscosity, and hence reduce fluidity, and adversely affect the feeding of castings. Surface oxide skins can significantly increase the apparent surface tension of melts and increase the possibilities of forming cold shut, flow marks, and misruns.
In order to minimize and eventually eliminate the oxides in the final cast aluminum products, it is desired to identify not only the oxide types, but also the time when they are formed particularly for young oxides in the casting process. This will help foundrymen to design and optimize gating/riser system, filtration, and fill profile more effectively.
Although there is a strong practical need for characterizing aluminum oxides, no reliable method or technique has yet been reported.